Studies are continuting on the structure and biological activity of salt-soluble chromatin obtained from rabbit thymus nuclear lysates via an endogenous nuclease reaction. A modified system using buffer-washed lysates and the inclusion of proteolytic inhibitors such as leupeptin has been developed. The chromatin will be fractionated by sampling the endogenous reaction at different times and, in addition, by simple centrifugation of purified chromatin. The various fractions are to be characterized as to protein profile by gel electrophoresis; chromatin conformation will be assayed by circular dichroism. The binding and initiation of rabbit thymus RNA polymerase to the various chromatin fractions will be examined and correlated with protein profiles and chromatin conformation.